1. Field of the Invention
The present invention relates to a voltage control oscillator and a quadrature modulator, and, more particularly is suitably applied to a voltage control oscillator and a quadrature modulator that can obtain oscillation outputs for four phases different from one another by 90°.
2. Description of the Related Art
In a communication system employing quadrature modulation, local oscillation signals having phases different from one another by 90° are used in both a modulating circuit in a transmitting unit and a demodulating circuit in a receiving unit. To generate such local oscillation signals, in some case, a quadrature voltage-controlled oscillator (QVCO) is used.
For example, Japanese Patent Application Laid-Open No. 2004-120728 discloses, in an LC resonant oscillator in which a plurality of capacitative elements are connected to output nodes, voltage for selecting an oscillation frequency band is applied to terminals on the opposite side of the capacitative elements, and the oscillation frequency band can be changed stepwise according to the selection voltage, a method of configuring the capacitative elements with variable capacitative elements.
However, in the QVCO in the past, the phases of the local oscillation signal are shifted from one another by 90° according to mutual inductance of an inductor connected to a source side of a certain field effect transistor and an inductor connected to a drain side of another field effect transistor. Therefore, in the QVCO in the past, when drain-to-source capacitance of the field effect transistors fluctuates, a phase error increases.